In the manufacture of articles from rubbery, viscoelastic and plastic working materials, sometimes called plastic stock, it is common practice to filter the working materials. This may be done, for example, in a reclaiming process, to prepare re-claimed materials for use in a product; it may also be done, for example, in an extrusion apparatus, to make certain that no foreign matter will be introduced into an extrusion die.
An extrusion process is a continuous process in which the quality and quantity of the extrudate is proportional to the stability and consistency of the melt. A change in operating conditions changes the quality or quantity or both of the end product. The practice of filtering in an extrusion process results in a change of operating conditions because of the accumulation of contaminants on the filter. When the filter is clean a 1500 psi pressure drop might appear at the start of the process thus establishing a given quality and rate. As time goes on and the screen collects contaminants from the melt, the pressure drop increases, the rate of production decreases, the melt temperature increases, and thus the quality of the product is changed. When the pressure drop reaches 4,000 psi, for example, and the dirty filter is changed for a clean one, the pressure is reduced back to 1500 psi instantaneously. This causes a severe transient to the process requiring stabilization time and operator attention. During this stabilization time inferior product may be produced.
To minimize these changes in operation slide plate filters have been utilized. One of the earliest slide plate filters is shown in U.S. Pat. No. 42,814 issued to Cowen in 1900. Slide plate filters according to Cowen are still in use today. Other forms of slide plate filters which like Cowan are intended to be athwart an oncoming stream of plastic stock (the "melt stream") are shown in Garrahan's U.S. Pat. No. 1,195,576; Voight U.S. Pat. No. 3,684,419 and Paquette U.S. Pat. No. 3,797,665. In general, the area of filter surface which is presented across the melt stream is essentially the same as a right cross section of that stream.
It is common practice to use filters of the type described, in apparatus by which the plastic stock is pressurized; in some such apparatus the pressure applied may reach 10,000 pounds per square inch or more. There is, therefore, a need to seal the apparatus against leakage of plastic stock from the melt stream along surfaces of the slide plate. Paquette describes one such sealing mechanism.